Gases I. Physical Properties

A. Properties of Gases Expand to completely fill their container Take the shape of their container Low density – mass divided by volume. Take up much more space than their mass. Much less than solid or liquid state Compressible Mixtures of gases are always homogeneous

C. Characteristics of Gases Gases expand to fill any container. random motion, no attraction Gases are fluids (like liquids). except no attraction b/w particles. Gases have very low densities. low volume = lots of empty space

C. Characteristics of Gases Gases can be compressed. low volume = lots of empty space Gases undergo diffusion & effusion

Factors that affect Gases.. Pressure – the force that gas exerts on a given area of a container. Volume – the space inside a container holding the gas. Concentrations (Moles) – number of molecules. We convert to mass using molecular weight. Temperature – the average speed of the gas particles.

E. Pressure Which shoes create the most pressure?

Pressure Pressure of air is measured with a BAROMETER (developed by Torricelli in 1643) Hg rises in tube until force of Hg (down) balances the force of atmosphere (pushing up). This is how a straw works. A decrease in pressure inside the straw yields to the atmospheric pressure pushing down and moves the liquid up.

Measuring Pressure of a Trapped Gas (cont.)

E. Pressure KEY UNITS AT SEA LEVEL 101.325 kPa (kilopascal) 1 atm 760 mm Hg 760 torr 14.7 psi

Pressure Conversions A) 2.5 atm = ?kPa B) 200 torr = ? mmHg C) 30 psi = ?atm A) 1200 torr = ? atm

D. Temperature Always use absolute temperature (Kelvin) when working with gases. ºF ºC K -459 32 212 -273 100 273 373 K = ºC + 273

Temperature Conversions A) 10 ºC = ? K B) 78 ºF = ? ºC C) 300 K = ? ºC D) 80 ºF = ? K

Standard Temperature & Pressure F. STP STP Standard Temperature & Pressure 273 K 1 atm

Boyles’ Law If the temperature remains constant, the volume and pressure vary inversely i.e. if P ↑, then V↓; and vice versa P1 V1 = P2 V2

Boyle’s Law (cont.)

Boyles’ Law Example If a gas has a volume of 200ml at 800mmHg pressure, calculate the volume of the same gas at 765mmHg. P1= 800mmHg V1= 200ml P2 =765mmHg V2=? Formula P1 V1 = P2 V2

Formula P1 V1 = P2 V2 Plug in values 800mmHg x 200ml = 765mmHg x V2 Solve V2= 800mmHg x 200ml 765mmHg V2 = 209.15ml

Charles’ Law If the pressure remains constant, the volume and temperature vary directly i.e. if T ↑, then V↑; and vice versa V1 V2 T1 T2 =

Charles’ Law (cont.)

Charles’ Law Example The volume of a gas at 20°C is 500ml. Find its volume at standard temperature if the pressure is held constant. T1= 20°C +273 V1= 500ml T2 = 0°C +273 V2=?

Formula V1 = V2 T1 T2 Plug in values 500ml V2 293K 273K Solve V2= 273K x 500ml 293K V2 = 465.87ml =

Gay Lussac’s Law If the volume remains constant, the temperature and pressure vary directly i.e. if P ↑, then T↑; and vice versa P1 P2 T1 T2 =

Pressure and Temperature (cont.)

Gay-Lussac’s Example A steel tank contains a gas at 27°C and a pressure of 12 atm. Determine the gas pressure when the tank is heated to 100°C. T1= 27°C +273 P1= 12 atm T2 = 100°C +273 P2=?

P1 = P2 T1 T2 Plug in values 12 atm P2 300K 373K Solve P2= 373K x 12atm 300K P2 = 14.92 atm =

No, it’s not related to R2D2 Combined Gas Law The good news is that you don’t have to remember all three gas laws! Since they are all related to each other, we can combine them into a single equation. P1 V1 P2 V2 = T1 T2 No, it’s not related to R2D2

Combined Gas Law If you should only need one of the other gas laws, you can cover up the item that is constant and you will get that gas law! P1 V1 P2 Boyle’s Law Charles’ Law Gay-Lussac’s Law V2 T1 T2

Combined Gas Law Problem A sample of helium gas has a volume of 0.180 L, a pressure of 0.800 atm and a temperature of 29°C. What is the new temperature(°C) of the gas at a volume of 90.0 mL and a pressure of 3.20 atm? Set up Data Table P1 = 0.800 atm V1 = .180 L T1 = 302 K P2 = 3.20 atm V2= .090 L T2 = ??

Calculation P1 = 0.800 atm V1 = 180 mL T1 = 302 K P2 = 3.20 atm V2= 90 mL T2 = ?? P1 V1 P2 V2 = P1 V1 T2 = P2 V2 T1 T1 T2 T2 = P2 V2 T1 P1 V1 T2 = 3.20 atm x 90.0 mL x 302 K 0.800 atm x 180.0 mL T2 = 604 K - 273 = 331 °C = 604 K

Learning Check A gas has a volume of 675 mL at 35°C and 0.850 atm pressure. What is the temperature in °C when the gas has a volume of 0.315 L and a pressure of 802 mm Hg?

Ideal Gas Law By combining the proportionality constants from the gas laws we can write a general equation. R is called the gas constant. The value of R depends on the units of P and V. Generally use R = 0.082 when Pressure is in atm and Volume is in L PV = nRT 14

T = Temperature (in Kelvin) Ideal Gas Law PV=nRT P = Pressure (In atm) V = Volume (In Liters) n = Moles (In mol) R = Gas Constant = 0.082 T = Temperature (in Kelvin)

Example 1 A 7.50 liter sealed jar at 18 °C contains 0.125 moles of oxygen and 0.125 moles of nitrogen gas. What is the pressure in the container?

Example 2 A 4.0 liter container has two gases inside, neon and argon. It is known that at 18 °C, the total pressure of the combined gases is 0.850 atm. If it is known that there are 0.100 moles of neon in the container, how many moles of argon are in the container?